Transmitter designs for wireless devices typically are constrained to meet specific quality of service (QoS) requirements relating to certain measurable characteristics of transmitted radio frequency (RF) signals. These QoS requirements may be specified by various standards or manufacturer-imposed specifications. For example, a standard or specification may set forth not-to-exceed values for the adjacent- and/or alternate-channel leakage ratio (ACLR), block error rate (BLER), and/or bit error rate (BER), among other things. Manufacturer adherence to these standards and specifications helps to ensure high signal quality and end-user satisfaction.
RF transmitters for some types of wireless devices are designed to enable the transmitted signal power to be adjusted dynamically. For example, power control methods are implemented in RF transmitters for “third generation” (3G) wireless devices, such as a variety of 3G Wideband Code Division Multiple Access (W-CDMA) transmitters. In such transmitters, the signal gain dynamically is adjusted (i.e., increased and decreased) based on various factors, such as a comparison between the then-current, received signal to interference ratio (SIR) and a target SIR, for example. In order to implement the dynamic power control, a W-CDMA transmitter may include a variable gain amplifier (VGA), which receives a gain control signal that may cause the VGA to apply increased or decreased gains to an outgoing RF signal to produce a gain-adjusted RF signal. The gain-adjusted RF signal is provided to an output power amplifier, which amplifies the gain-adjusted RF signal and provides the amplified signal to the device's antenna.
Traditional RF transmitters include the transmit digital sections, the transmit digital analog converter (DAC), and the modulators on a complementary metal oxide semiconductor (CMOS) die, and the variable gain amplifier constructed on a separate silicon-germanium (SiGe) die. Although these traditional transmitters may produce RF signals having adequate signal quality, the multiple-die architecture is a fairly expensive portion of the overall device manufacturing costs. In addition, the continuing industry trend is toward reducing device sizes, and the multiple-die architecture places limitations on the ability to reduce the size of the device's RF transmitter. Accordingly, what are needed are methods and apparatus for automatic gain control for transmitted RF signals, which produce signals having acceptable signal quality, and which permit reductions in device manufacturing costs and/or device sizes.